1. Field of the Invention
This invention relates to a quartz glass crucible for pulling a semiconductor single crystal such as silicon.
More specifically, the present invention relates to a quartz glass crucible having two layers in its structure, which can decrease defects in pulled silicon single crystals. In the single crystal pulling process, such crucibles are used as a container for melted semiconductor materials.
2. Description of Related Art
Conventionally, in producing a single crystal such as a single crystal silicon semiconductor material, the so-called "Czochralski" method has been widely adopted. According to that method, polycrystalline silicon is melted in a vessel, and a seed crystal is dipped at its tip end portion into the melt. Then the seed is pulled while it is being rotated so that a single crystal is grown on the seed with the same crystallographic orientation. A vessel adapted for the pulling of a single crystal is made in the form of a quartz crucible. Quartz crucibles are classified in accordance with their external appearance, which is determined by the differences in the production methods. There are two basic categories, namely, transparent crucibles having a relatively low bubble content, and opaque crucibles having a high content of minute bubbles to provide an opaque appearance. The latter crucible will hereinafter be referred to as the "opaque crucible".
The opaque crucible is produced by a process in which powders of quartz are introduced into a mold and accumulated to form a layer along the inner surface of the mold. The layer of quartz powder is then heated at the inner surface thereof while the mold is rotated, to produce a crucible of a glass having a relatively high bubble content. The opaque quartz crucible is advantageous in that it has a higher strength when compared with a transparent crucible, and it is relatively easy to form a crucible of a large size. Further, minute bubbles contained in the opaque crucible contribute to uniformize the thermal distribution. For the reasons stated above, the opaque crucible has been widely used in practice.
In the above-described conventional quartz glass crucible, which consists of a totally opaque body, the bubbles in the crucible body increase in volume during the step of pulling the single crystal under a reduced pressure at a high temperature. Consequently, the specific gravity of the crucible changes, and the crucible is deformed so as to change the liquid level of the melt in the crucible. Sometimes, the bubbles in the crucible body are ruptured, with the result that the glass layer of the crucible positioned above the liquid level peels and the peeled particles fall into the melt in the crucible. As a result, the contact area between the crucible and the melt is increased, causing the concentration of dissolved oxygen to increase. Accordingly, the pull yield is decreased, and then the quality of the semiconductor wafers is decreased.
U.S. Pat. No. 4,956,208 teaches the formation of a substantially bubble-free inner layer on an outer substrate layer, by forming an atmosphere of high temperature gas and supplying a metered quantity of quartz powders to the high temperature gas atmosphere. It is intended to have the quartz powders be at least partly molten and directed toward an inner surface of the substrate layer so as to be adhered thereon.
U.S. Pat. No. 4,528,163 teaches the formation of a crucible substrate with an outer layer made of powders of natural quartz and an inner layer made of powders of synthetic quartz. The crucible substrate is then heated at the inner surface to form a thin smooth layer of an amorphous nature. In the specification of the '163 patent, the inner layer comprises synthetic "crystalline" quartz. The inner layer formed in accordance with the teaching of this patent should be considered to contain a noticeable quantity of bubbles or voids.
U.S. Pat. No. 4,416,680 and U.S. Pat. No. 4,632,686 teach the idea of decreasing the bubble content by applying a suction pressure to the external surface of the quartz crucible while the crucible is being heated. However, the processes taught by these patents are not particularly effective to decrease the bubble content because the bubbles are subjected to a substantial resistance in passing through the layer of quartz glass.
In the practice of pulling silicon single crystals, the quality control of the pulled single crystal, even during pulling, is significant. It is said that some microdefects, such as so-called oxidation stacking fault (OSF) or bulk micro defect (BMD), are mostly due to the thermal history of the single crystal in a high temperature area near the crucible after it was crystallized. The pulling of silicon single crystals is carried out usually with the apparatus shown in FIG. 1. With reference to FIG. 1, some upper portion of quartz crucible 21 is positioned above the carbon holder 22, in order to make it easy to handle the quartz crucible. In such a construction, if there is a transparent layer 25 which tends to transfer heat easily, the heat coming from the heater 23 which surrounds the crucible will become more than that of an opaque crucible, and the heat from the melt contained in the crucible will transfer via the transparent layer 25 and will emit from the top end of the crucible. The just-crystallized silicon crystal 27 which is being pulled receives such undesirable heat emissions. Additionally, since the heat escapes from the melt through the transparent layer to the top end of the layer, which terminates freely, control of the melt temperature becomes difficult. As a result of these problems, the OSF and BMD increase, and the yield of good single crystal is decreased.
Therefore, there is a need for providing a quartz crucible which can overcome such problems.